Method and apparatus improving vision and the resolution of retinal images

ABSTRACT

A method of and apparatus for improving vision and the resolution of retinal images is described in which a point source produced on the retina of a living eye by a laser beam is reflected from the retina and received at a lenslet array of a Hartmann-Shack wavefront sensor such that each of the lenslets in the lenslet array forms an aerial image of the retinal point source on a CCD camera located adjacent to the lenslet array. The output signal from the CCD camera is acquired by a computer which processes the signal and produces a correction signal which may be used to control a compensating optical or wavefront compensation device such as a deformable mirror. It may also be used to fabricate a contact lens or intraocular lens, or to guide a surgical procedure to correct the aberrations of the eye. Any of these methods could correct aberrations beyond defocus and astigmatism, allowing improved vision and improved imaging of the inside of the eye.

REFERENCE TO RELATED APPLICATION

The present application is a division of U.S. patent application Ser.No. 11/199,174 filed Aug. 9, 2005, which is a continuation of U.S.patent application Ser. No. 10/078,163 filed Feb. 20, 2002, now U.S.Pat. No. 6,948,818, which is a continuation of U.S. patent applicationSer. No. 09/628,690, filed Jul. 28, 2000, now U.S. Pat. No. 6,379,005,which is a continuation of U.S. patent application Ser. No. 09/346,309,filed Jul. 2, 1999, now U.S. Pat. No. 6,095,651, which is a continuationof U.S. patent application Ser. No. 09/071,794, filed May 4, 1998, nowU.S. Pat. No. 5,949,521, which is a continuation of U.S. patentapplication Ser. No. 08/772,977, filed Dec. 23, 1996, now U.S. Pat. No.5,777,719. The disclosure of all of the above applications are herebyincorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention is directed to a method of and an apparatus forimproving vision and the resolution of retinal images. Moreparticularly, the present invention is directed to a method of and anapparatus for measuring and correcting the wave aberration of the eyesuch that the measured data can be used to develop corrective opticalelements for improving the optical quality of the eye.

Despite significant advances in spectacle and contact lens design,current ophthalmic lenses still can only correct defocus andastigmatism. Spectacles and contact lenses leave uncorrected additionalaberrations such as spherical aberration, coma, and a host irregularaberrations. These high order aberrations of the eye not only blurimages formed on the retina, which impairs vision, but also blur imagestaken of the living human retina. There have been two obstacles thatprevent the use of specially-designed optical elements to correctaberrations beyond defocus and astigmatism in the eye. First,quantitative measurement of the irregular aberrations of the eye has notbeen possible. Second, a mechanism to correct the monochromaticaberrations of the eye other than defocus and astigmatism has not beendemonstrated.

Subjective refractive methods of optometrists and objectiveautorefractors measure defocus and astigmatism only. They cannot measurethe complete wave aberration of the eye, which includes all aberrationsleft uncorrected by conventional spectacles. The objective aberroseopedisclosed by Walsh et al. in the Journal of the Optical Society ofAmerica A, Vol. 1, pp. 987-992 (1984) provides simultaneous waveaberration measurement of the entire pupil but cannot sample the pupilwith a spacing finer than about 0.9 mm (See Charman in Optometry andVision Science, Vol. 68, pp. 574-583 (1991)). Moreover, rapid, automatedcomputation of the wave aberration has not been demonstrated with thismethod.

Recently, one of the co-inventors herein, together with others,developed an apparatus to measure the wave aberration of the eye. In areport entitle “Objective measurement of wave aberration of the humaneye with the use of a Hartmann-Shack wave-front sensor”, Liang et al.,J. Opt. Soc. Am. A., volume 11, number 7, pp. 1-9, July 1994, thedisclosure of which is incorporated by reference herein, the authorsdisclosed a Hartmann-Shack wavefront sensor that they used to measurethe wave aberrations of the human eye by sensing the wavefront emergingfrom the eye produced by the retinal reflection of a focused light beamon the fovea. Using the system disclosed therein, the authors were ableto measure only up to fourth order polynomial functions. The wavefrontfitting with polynomials up to fourth order does not provide a completedescription of the eye's aberrations. That description is generallyinsufficient to accurately compute the optical performance of the eye.This instrument was not equipped to remove unwanted light reflected fromother surfaces, such as lenses and the cornea of the eye.

There has also been a previous attempt to correct the monochromaticaberrations of the eye beyond defocus and astigmatism, with the goal ofimproving the axial resolution of the confocal scanning laserophthalmoscope. Bartsch et al., in Vision Science and its Applications,1994, Technical Digest Series, Vol. 2 (Optical Society of America,Washington, D.C.) pp. 134-137 (1994) used a fundus contact lens to nullthe refraction at the first surface of the cornea. That approach,however, suffers from the fundamental problem that the wave aberrationof the eye depends on the combined effects of refractive indexvariations throughout the eye's optics. Possibly for that reason, anattempt to use a fundus contact lens to increase the axial resolution ofa confocal scanning laser ophthalmoscope showed only modest improvement.

Another approach is to use a deformable mirror, a device that hassuccessfully compensated for atmospheric turbulence in ground-basedtelescope. A deformable mirror was previously proposed for use in aconfocal laser scanning ophthalmoscope in conjunction with the human eyein U.S. Pat. No. 4,838,679 to Billie, but no method to measure the waveaberration of the eye was proposed or disclosed. Dreher, Bille, andWeinreb, in Applied Optics, Vol. 28, pp. 804-808 demonstrated the onlyusage of a deformable mirror for the eye, but only corrected theastigmatism of the eye, which is no better than the correction providedby conventional ophthalmic lenses. The use of an optical element tocorrect monochromatic aberrations higher than second order has neverbeen achieved. In both those systems, no appropriate method formeasuring the eye's high order aberrations was disclosed. Bille et al.,in Noninvasive Diagnostic Techniques in Ophthalmology, edited byMasters, B. R., Springer-Verlag, pp. 528-547 (1990) proposed the use ofa wavefront sensor in conjunction with a deformable mirror, but aworking system was never disclosed or realized.

SUMMARY AND OBJECTS OF THE INVENTION

In view of the foregoing, it is apparent that there exists a need in theart for a method of and an apparatus for producing ophthalmic opticalelements that provide improved or supernormal vision over that which iscurrently available, as well as high resolution retinal images. It is,therefore, a primary object of the present invention to provide a methodof and an apparatus for accurately measuring higher order aberrations ofthe eye and for using the data thus measured to compensate for thoseaberrations with a customized optical element.

It is also an object of the present invention to provide an improvedwavefront sensor which rejects light reflected from structures otherthan the retina and which is capable of providing a complete measurementof the eye's aberrations.

It is a further object of the present invention to utilize such animproved wavefront sensor in combination with a deformable mirror tocorrect the wave aberration in a feedback manner such that the subjectachieves normal or supernormal vision.

It is likewise a primary object of the present invention to provide amethod of and an apparatus for producing high resolution retinal imageswhich allow the imaging of microscopic structures the size of singlecells in a human retina.

Briefly described, these and other objects of the invention areaccomplished by providing a system for receiving light reflected from aretina of an eye. The wavefront in the plane of the pupil is recreatedin the plane of a lenslet array of a Hartmann-Shack wavefront sensor.Each of the lenslets in the lenslet array is used to form an aerialimage of the retinal point source on a CCD camera located adjacent tothe lenslet array. The wave aberration of the eye, in the form of apoint source produced on the retina by a laser beam, displaces each spotby an amount proportional to the local slope of the wavefront at each ofthe lenslets. The output from the digital CCD camera is sent to acomputer which then calculates the wave aberration and provides a signalto a deformable mirror. Following an iterative procedure, the deformablemirror ultimately acquires a shape that is identical to the waveaberration measured at the outset, but with half the amplitude, Thisdeformation is the appropriate one to flatten the distorted wavefrontinto a plane wave, which improves image quality.

In its method aspects, the system of the present invention, using thecomputer, first acquires the CCD image, as described above. Then, thecomputer computes the centroid of the spot formed by each of thelenslets of the wavefront sensor. Shifts in each focus spot in the x andy directions are calculated and then used as the slope data to fit withthe sum of the first derivatives of 65 Zernike polynomials, using aleast squares procedure, to determine the weight for each polynomial.

Then, the Zernike polynomials are weighted with the calculatedcoefficients. The 65 polynomials in the wavefront fit include allZernike modes with radial power less than or equal to 10, except for thepiston term.

The weighted Zernike polynomials are then added together, to result inthe reconstructed wave aberration. The wave aberration is then evaluatedat the locations of the actuators of a deformable mirror in order toproduce the correction signal which is sent by the computer to thewavefront compensation device or deformable mirror as discussed above.Such a feedback loop continues to receive the reconstructed waveaberration results, feeding back an appropriate correction signal, untilthe RMS of the reconstructed wave aberration signal reaches anasymptotic value, at which point, the deformable mirror has beendeformed such that it will compensate for all the detected aberrationsof the eye.

When the reconstructed wave aberration signal reaches its asymptoticvalue, the final aberration signal (including all of the previouslygenerated signals up until the RMS error reaches the asymptotic value)can be used to produce contact lenses to correct for all of themonochromatic aberrations of the human eye or for surgical procedures.

The present invention can also be used to provide high resolution imagesof the retina. The system for producing such images uses a krypton flashlamp which is designed to illuminate a retinal disk to provide an imageof the retina which is reflected by the deformable mirror onto a lensand through an aperture such that the reflected image of the retina isfocused onto a second CCD camera. The signals generated by the cameraare acquired in a manner similar to that described above in connectionwith the first CCD camera and are stored for later use in the computer.

With these and other objects, advantages, and features of the inventionthat may become hereinafter apparent, the nature of the invention may bemore clearly understood by reference to the following detaileddescription of the invention, the appended claims and to the drawingsattached herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the system of the present invention;

FIG. 2 is a diagram of a flow chart of a method for the presentinvention for use in fabricating contact lenses or providing red imagesusing the apparatus shown in FIG. 1;

FIGS. 3 a-3 d show the wave aberration and the corresponding pointspread functions before and after adaptive compensation using the systemof the present invention, respectively; and,

FIG. 4 shows the Zernike decomposition of the wave aberration of the eyebefore and after adaptive compensation, showing that the invention cancorrect not only second order aberrations (defocus and astigmatism) butalso high-order aberrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown, in schematic diagram form, theapparatus of the present invention which can be used to both improvevisual performance and to provide high resolution retinal images ofeyes. The apparatus of the present invention, as shown in FIG. 1,measures the aberrations of the eye using a Hartmann-Shack wavefrontsensor and then corrects them in a closed feedback loop with acompensating optical component such as a deformable mirror.

To measure the eye's wave aberration, a point source is produced on theretina by a laser 102. The light from the laser 102, is controlled by ashutter (not shown). The laser light passes through a spatial filter 104and is collimated by an achromatic doublet lens 106. The collimatedlaser beam is reflected by the polarizing beamsplitter 110, passesthrough the lenses 112 and 116, and is incident onto a deformable mirror118. The laser beam reflected from the deformable mirror 118 is focusedby the lens 120, passes through the lens 122 and the second beamsplitter124 and reaches the eye 100 at a diameter of about 1.5 mm at the pupil.The lens of the eye 100 focuses the laser beam on its retina 101.Possible myopia or hyperopia of the tested eye is corrected by movementof the eye 100 and the lens 122 with respect to the lens 120.

The light reflected from the retina 101 forms a distorted wavefront atthe pupil, which is recreated in the plane of the deformable mirror 118by the lenses 122 and 120, and also in the plane of the lenslet array ofa Hartmann-Shack wavefront sensor 148 by the lenses 116 and 112. Thepupil is conjugate with a two-dimension array of lenslets 148. Each ofthe lenslets in the array 148 forms an aerial image of the retinal pointsource on the CCD camera 146. The CCD camera 146 sends the digitizedimage to the computer 150 where it is stored in RAM or on magnetic orother storage media.

Aberrations displace each spot by an amount proportional to the localslope of the wavefront at each lenslet. The slope of the wavefront canbe determined from the displacement of these spots relative to a knownaberration-free reference. The wavefront sensor 154 mares the localslopes in both the x and y directions at 217 locations simultaneouslyacross the dilated pupil. From the array of slopes, the wave aberrationis reconstructed as described later herein.

The beamsplitter 110 which forms part of the wavefront sensor 154 ispreferably configured as a polarizing beamspiitter in order to removeunwanted light reflected from the first surface of the cornea of the eye100 and other optical elements, such as lenses 112, 116, 120, and 122.Such light would otherwise interfere with the operation of the wavefrontsensor 154 and would make automatic procedures for measuring the waveaberration more difficult.

The wavefront sensor 154 of the present invention provides a much morecomplete measurement of the aberrations of the eye than has beenpossible before in part because a large number of samples in the pupilof the eye are used. Using the apparatus of the present invention, ithas been found that the eye 100 cont complex aberrations that am notcorrected by spectacles or contact lenses, and that such aberrations aresignificant when the pupil is large. Even aberrations of order 5 andgreater, which the present wavefront sensor 154 is the first to measure,has significant impact on the eye's optical quality. Moreover, thehigher order aberrations are stable over time. Such stability isimportant because it shows that the aberrations can be corrected using astatic optical corrector, such as a custom-made contact lens.

Once the wave aberration is measured, as described above, it iscompensated with a device conjugate with the pupil of the eye 100 thatcan control the phase of the light at different points in the pupil. Aswill be further described in detail in connection with FIG. 2, thecomputer 150, under software control, produces a correction signal whichis fed back to the deformable mirror 118. The deformable mirror 118,which is available as model no. RX-170407-C from Xinetics, Inc., isconjugate with the pupil of the eye 100 and will be deformed tocompensate for the wave aberration of the eye. The deformable mirror 118contains an aluminized glass faceplate with 37 actuators mounted in asquare array on the back surface. Alternatively, a liquid crystaldevice, a micro-machined mirror, a bimorph mirror, or other suitabledevice could be used in place of the deformable mirror 118.

The apparatus of FIG. 1 is used as follows. The computer 150 starts themeasurement process by opening the pathway of the laser beam from thelaser 102. The light returning from the retina 101 is used to measurethe wave aberration. The Hartmann-Shack wavefront sensor 148 provides ameasurement of the eye's wave aberration, from which the deformablemirror 118 is driven to correct it. The process is repeated with thecomputer 150 generating suitable signals to cause the deformable mirror118 to continue deforming until the RMS error in the measured waveaberration reaches an asymptotic value, although other criteria might beused. At that point, the deformable mirror 118 has taken the appropriateshape to provide wavefront compensation for the aberrations of the eye.

Also at that point, the final correction signal generated by thecomputer 150 can be provided to a contact lens fabrication system 152used to fabricate contact lenses which would duplicate the wavefrontcompensation characteristics of the deformable mirror 118. As would beobvious to those of ordinary skill in the art, such contact lenses wouldbe custom ground for each eye for which compensation is to be provided.

Turning now to FIG. 2, there is shown in diagrammatic flow chart formthe steps performed by the software resident in the computer 150 forutilizing the data obtained from the CCD device 146. At Step 1, thedigital image is acquired by the computer 150 from the CCD camera. Eachimage consists of 512 pixels by 512 pixels at 12 bits. Then, at Step 2,the computer 150 computes the centroid of the spot formed by eachlenslet of the wavefront sensor. The centroid of each spot specifies itsposition. By comparing the positions of corresponding spots in areference pattern and in the pattern obtained from the eye, the shiftsin each focus spot in both the x and y directions can be calculated.

Next, at Step 3, the slope data are fit with the sum of the firstderivatives of 65 Zernike polynomials, using a least squares procedureto determine the weight for each polynomial, similar to that discussedby Liang et al. in their report referenced above, though their methodincluded no polynomials beyond fourth order.

Then, at Step 4, the Zernike polynomials are weighted with thecoefficients calculated at Step 3. The 65 polynomials in the wavefrontfit include all Zernike modes with radial power less than or equal to10, except for the piston term. The first order Zernike modes are thelinear terms. The second order modes are the quadratic terms, whichcorrespond to the familiar aberrations, defocus, and astigmatism. Thethird order modes are the cubic terms, which correspond to the coma andcoma-like aberrations. The fourth order modes contain sphericalaberrations as well as other modes. The fifth to tenth order modes arethe higher-order, irregular aberrations. Local irregularities in thewavefront within the pupil are represented by these higher-order Zernikemodes.

The weighted Zernike polynomials are added together at Step 5 to obtainthe reconstructed wave aberration. The wave aberration is evaluated atthe locations of the actuators of the deformable mirror 118 to produce acorrection signal at Step 6, which is then sent by the computer 150 tothe wavefront compensation device which, as shown in FIG. 1, ispreferably a deformable mirror 118. The feedback loop continues toreceive the reconstructed wave aberration results, feeding back anappropriate correction signal until the RMS error in the reconstructedwave aberration signal reaches an asymptotic value. At that point, thedeformable mirror 118 has been deformed such that, when the eye looksthrough it, it will compensate for all of the detected aberrations ofthe eye 100.

As is known to those of ordinary skill in the art, spectacles andcontact lenses are available to correct vision, but they only correctdefocus and sometimes astigmatism in the eye. Sven normal eyes haveother aberrations that are not corrected by conventional spectacles orcontact lenses. The wavefront sensor 154 of the present invention iscapable of automatically measuring those aberrations. Thus, using theadaptive optics system of FIG. 1 allows correction of monochromaticaberrations of the human eye beyond defocus and astigmatism.Accordingly, the adaptive optics system of FIG. 1 provides unprecedentedoptical quality in human eyes, both for improved vision and forproviding sharper pictures of the retina.

FIGS. 3 a-3 d are graphs of data from the wavefront sensor showing thatthe adaptive optics system of the present invention successfullycorrects the aberrations of the eye. FIGS. 3 a and 3 b show, for onesubject, the wavefront error versus pupil position without adaptivecompensation and with adaptive compensation, respectively, using thepresent invention. In a test in which four subjects were measured,adaptive compensation reduced the peak to valley wavefront error acrossa 6 mm pupil by a factor of 4, on average.

FIGS. 3 c and 3 d show the point spread function (PSF), computed fromthe wave aberration, without adaptive compensation and with adaptivecompensation, respectively. Adaptive compensation increased the Strehlratio, which is proportional to the peak intensity of the PSF, from 0.09to 0.47. The average increase for all four eyes measured was nearly 4fold, from 0.06 to 0.23. After compensation, the PSF for the subjectshown has a full-width at half height (FWHH)of 2.0 microns, close to thevalue of 1.9 microns expected from diffraction alone. The presentinvention provides what is believed to be the best optical quality everachieved in the human eye.

FIG. 4 is a graph of the RMS wavefront error, averaged across 4 eyes,associated with each Zernike order of the wave aberration. The RMSwavefront error is reduced by adaptive compensation for Zernike ordersup to fifth order. Spectacles, contact lenses, and one previous attemptto use a deformable mirror to correct the eye's aberrations have onlycorrected second order aberrations (defocus and astigmatism).

Once the correction has been achieved, the apparatus of the presentinvention can be used to provide the subject with supernormal retinalimage quality, in which case the subject views visual imagery throughthe deformable mirror 118. A six-fold improvement in vision, as measuredwith contrast sensitivity for fine gratings, for individuals lookingthrough the present apparatus has been achieved. Individuals can seevery high frequency gratings through the adaptive optics system that areabove the resolution limit in normal viewing without adaptive optics.

Alternatively, once the correction has been achieved, the apparatus ofthe present invention shown in FIG. 1 can also be used to provide highresolution images of the retina 101. A krypton flash lamp 138 is imagedonto the eye's pupil by the lenses 134, 132, 130, and 120 after passingthrough the beamsplitter 124. The lamp 138 delivers a 4 msec flash,thereby illuminating a retinal disk on the retina 101 that is 1 degreein diameter. A narrow band interference filter 136 shapes the spectraloutput of the lamp 138.

The image of the retina 101 passes through the beamsplitter 124 and thelenses 122 and 120, is reflected by the deformable mirror 118, which hasalready been shaped to compensate for the eye's aberrations, and passesthrough the lens 116. The light is then reflected by the mirror 114 ontothe lens 140 and through an aperture 141, where the reflected image isfocused by the lens 142 onto the CCD device 145. The electrical signalscorresponding to the light captured by the CCD camera 145 may beacquired in a manner similar to the light acquisition system whichacquires the data from the CCD camera 146 and then be stored for lateruse in the computer 150. The images may then be shown on a monitor (notshown) connected to the computer 150 and/or sent to a suitable printingdevice (not shown) connected to the computer 150.

Using the apparatus of the present invention, single cells in the livinghuman retina have been routinely resolved for the first time. Theadaptive optic system of the present invention provides a non-invasivetechnique with which to study the normal and pathological living retinaat a microscopic spatial scale. The theoretical limit of the transverseresolution of fundus imaging is proportional to the diameter of thedilated pupil. If adaptive compensation were complete for an 8 mm pupil,for example, the transverse resolution would be increased 3.2 times overthat for a 2.5 mm pupil, a typical pupil diameter used by currentlyavailable fundus cameras.

The axial resolution, which is critical in optical in-depth sectioningof the retina, grows as the square of the pupil diameter. Therefore,complete adaptive compensation across an 8 mm pupil could theoreticallyincrease the axial resolution of a confocal ophthalmoscope by a factorof 10 over an instrument with a 2.5 mm exit pupil. The full-width athalf-height (FWHH) of the PSF in depth is 27 microns, which approachesthat of optical coherence tomography, but provides the additionaladvantage of high transverse resolution not possible with opticalcoherence tomography alone.

The success of the apparatus of the present invention establishes thatit can also be used to improve vision by reshaping the eye's optics, asin laser refractive surgery, or by fabricating customized opticalelements such as contact lenses or intraocular lenses. As shown, FIGS. 1and 2, the final correction signal can be provided to a contact lens orintraocular lens fabrication system or laser refractive surgicalprocedure 152 instead of the deformable mirror 118. As would be obviousto those of ordinary skill in the art, such contact lenses, intraocularlenses, or surgical correction would be customized for each eye forwhich compensation is to be provided. Additional aberrations besidesdefocus and astigmatism could be corrected with either an open or aclosed loop system.

The invention described herein allows for the first complete descriptionof the aberrations of the human eye combined with a demonstrated methodto correct these aberrations. The applications of the present inventionare four-fold. First, the invention can be utilized to provide a moreaccurate measure of the aberrations of the unaided eye. Second, thepresent invention can be used to evaluate the benefit of varioustechniques to correct the aberrations of the eye, such as customglasses, custom contact lenses, and surgical procedures. Third, thepresent invention can be used to improve vision in optical instrumentssuch as telescopes and binoculars, custom glasses, custom contactlenses, and with surgical procedures such as photorefractive keratectomy(PRK). Fourth, the invention can be utilized to improve the axial andtransverse resolution of images of the living retina. Current funduscameras suffer from the problem that, in the prior art, the aberrationsof the eye limit its resolution, a limit that has remained since theinvention of the ophthalmoscope.

Although only a preferred embodiment is specifically illustrated anddescribed herein, it will be appreciated that many modifications andvariations of the present invention are possible in light of the aboveteachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

1.-12. (canceled)
 13. A method for fabricating contact lenses to correctat least the third order wave aberrations of the living eye, comprisingthe steps of: generating a reflected point source image of the retina ofsaid living eye; receiving said reflected point source image andconverting said point source image to corresponding digital signals;calculating wave aberrations of said eye so as to include at least thirdorder modes, using said digital signals; and receiving the calculatedwave aberrations for supplying at least one contact lens to providewavefront compensation for said at least third order wave aberrations ofsaid living eye.
 14. The method of claim 13, further including the stepof polarizing said reflected point source image to remove stray lightreflected from the cornea of said living eye prior to receiving saidreflected point source image. 15.-25. (canceled)